Memory element with a reactive metal layer

ABSTRACT

A memory cell including conductive oxide electrodes is disclosed. The memory cell includes a memory element operative to store data as a plurality of resistive states. The memory element includes a layer of a conductive metal oxide (CMO) (e.g., a perovskite) in contact with an electrode that may comprise one or more layers of material. At least one of those layers of material can be a conductive oxide (e.g., a perovskite such as LaSrCoO 3 -LSCoO or LaNiO 3 -LNO) that is in contact with the CMO. The conductive oxide layer can be selected as a seed layer operative to provide a good lattice match with and/or a lower crystallization temperature for the CMO. The conductive oxide layer may also be in contact with a metal layer (e.g., Pt). The memory cell additionally exhibits non-linear IV characteristics, which can be favorable in certain arrays, such as non-volatile two-terminal cross-point memory arrays.

RELATED APPLICATIONS

The application is a continuation of U.S. patent application Ser. No.14/167,694, filed Jan. 29, 2014, which is a continuation of U.S. patentapplication Ser. No. 13/272,985, filed Oct. 13, 2011, which issued onMar. 18, 2014 as U.S. Pat. No. 8,675,389, which is a continuation ofU.S. patent application Ser. No. 12/931,967, filed Feb. 15, 2011, nowabandoned, which is a continuation of U.S. patent application Ser. No.12/653,486, filed Dec. 14, 2009, which issued on Feb. 15, 2011 as U.S.Pat. No. 7,889,539, which is a continuation of U.S. patent applicationSer. No. 12/286,723, filed Oct. 1, 2008, which issued on Dec. 15, 2009as U.S. Pat. No. 7,633,790, which is a continuation of Ser. No.12/215,958, filed Jun. 30, 2008, which issued on Nov. 22, 2011, as U.S.Pat. No. 8,062,942, which is a divisional of Ser. No. 11/473,005 filedJun. 22, 2006, which issued on Jul. 1, 2008 as U.S. Pat. No. 7,394,679,which is a continuation of Ser. No. 10/773,549, filed Feb. 6, 2004,which issued on Jul. 25, 2006 as U.S. Pat. No. 7,082,052.

FIELD OF THE INVENTION

The present invention relates generally to computer memory, and morespecifically to memory fabrication.

DESCRIPTION OF THE RELATED ART

Memory can either be classified as volatile or nonvolatile. Volatilememory is memory that loses its contents when the power is turned off.In contrast, non-volatile memory does not require a continuous powersupply to retain information. Most non-volatile memories use solid-statememory devices as memory elements.

Certain conductive metal oxides (CMOs), for example, can be used assolid-state memory devices. The CMOs can retain a resistive state afterbeing exposed to an electronic pulse, which can be delivered through twoterminals. U.S. Pat. No. 6,204,139, issued Mar. 20, 2001 to Liu et al.,incorporated herein by reference for all purposes, describes someperovskite materials that exhibit such characteristics. The perovskitematerials are also described by the same researchers in“Electric-pulse-induced reversible resistance change effect inmagnetoresistive films,” Applied Physics Letters, Vol. 76, No. 19, 8 May2000, and “A New Concept for Non-Volatile Memory: The Electric-PulseInduced Resistive Change Effect in Colossal Magnetoresistive ThinFilms,” in materials for the 2001 Non-Volatile Memory TechnologySymposium, all of which are hereby incorporated by reference for allpurposes. However, the materials described in the U.S. Pat. No.6,204,139 patent are not generally applicable to RAM memory because theresistance of the material, when scaled to small dimensions, isconsidered to be too large to make a memory with fast access times.

In U.S. Pat. No. 6,531,371 entitled “Electrically programmableresistance cross point memory” by Hsu et al, incorporated herein byreference for all purposes, resistive cross point memory devices aredisclosed along with methods of manufacture and use. The memory devicecomprises an active layer of perovskite material interposed betweenupper electrodes and lower electrodes.

Similarly, the IBM Zurich Research Center has also published threetechnical papers that discuss the use of metal oxide material for memoryapplications: “Reproducible switching effect in thin oxide films formemory applications,” Applied Physics Letters, Vol. 77, No. 1, 3 Jul.2000, “Current-driven insulator-conductor transition and nonvolatilememory in chromium-doped SrTiO₃ single crystals,” Applied PhysicsLetters, Vol. 78, No. 23, 4 Jun. 2001, and “Electric currentdistribution across a metal-insulator-metal structure during bistableswitching,” Journal of Applied Physics, Vol. 90, No. 6, 15 Sep. 2001,all of which are hereby incorporated by reference for all purposes.

The discovery of the resistance-changing property of certain CMOs,however, is relatively recent and has not yet been implemented in acommercial memory product. There are continuing efforts to bring a truenon-volatile RAM (nvRAM) to market.

SUMMARY OF THE INVENTION

The present invention generally provides a multi-resistive state elementthat is created by treating a conductive element. One embodiment is amemory array that includes a plurality of two-terminal memory plugs.Each two-terminal memory plug is operable to change from a highresistive state to a low resistive state upon application of a firstwrite voltage and change from a low resistive state to a high resistivestate upon application of a second write voltage. Furthermore, each twoterminal memory plug includes a multi-resistive state element that has aconductive element and a reactive metal that reacts with the conductiveelement.

In yet another embodiment of the invention, the multi-resistive stateelement has a conductive element and a very thin layer of material thatis less than 200 Å thick deposited on the conductive element.

In some embodiments of the invention, the conductive element is aconductive metal oxide and either the reactive metal or the very thinlayer of material is Al, Ti, Mg, W, Fe, Cr, V, Zn, Ta or Mo. In otherembodiments of the invention, the memory plug includes a top electrodeand a bottom electrode. In yet other embodiments of the invention, thetop electrode is Pt.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may best be understood by reference to the followingdescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1A depicts a perspective view of an exemplary cross point memoryarray employing a single layer of memory;

FIG. 1B depicts a perspective view of an exemplary stacked cross pointmemory array employing four layer of memory;

FIG. 2A depicts a plan view of selection of a memory cell in the crosspoint array depicted in FIG. 1A;

FIG. 2B depicts a perspective view of the boundaries of the selectedmemory cell depicted in FIG. 2A;

FIG. 3 depicts a generalized representation of a memory cell that can beused in a transistor memory array;

FIG. 4 depicts an exemplary flow chart of various processing steps thatcould be involved in fabrication of a memory;

FIG. 5A is a chart of measurements across a multi-resistive state memoryelement after a layer of reactive metal has been deposited;

FIG. 5B is a chart depicting the double ramp voltage pulse used to forthe measurements of FIG. 5A;

FIG. 5C depicts a graph of one example of a non-linear I-Vcharacteristic of a discrete two-terminal memory element;

FIG. 6A depicts a schematic diagram of x-direction driver sets;

FIG. 6B depicts a schematic diagram of y-direction driver sets; and

FIG. 7 depicts an elevation view of an exemplary memory plug with fivelayers.

It is to be understood that, in the drawings, like reference numeralsdesignate like structural elements. Also, it is understood that thedepictions in the FIGS. are not necessarily to scale.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth toprovide a thorough understanding of the present invention. It will beapparent, however, to one skilled in the art that the present inventionmay be practiced without some or all of these specific details. In otherinstances, well known process steps have not been described in detail inorder to avoid unnecessarily obscuring the present invention.

The Memory Array

Conventional nonvolatile memory requires three terminal MOSFET-baseddevices. The layout of such devices is not ideal, usually requiring anarea of at least 8 f² for each memory cell, where f is the minimumfeature size. However, not all memory elements require three terminals.If, for example, a memory element is capable of changing its electricalproperties (e.g., resistivity) in response to a voltage pulse, only twoterminals are required. With only two terminals, a cross point arraylayout that allows a single cell to be fabricated to a size of 4 f² canbe utilized. U.S. patent application, “Cross Point Memory Array UsingMultiple Thin Films,” U.S. application Ser. No. 10/330,512, filed Dec.26, 2002, now issued U.S. Pat. No. 6,753,561, incorporated herein byreference in its entirety and for all purposes, describes such a device.

FIG. 1A depicts a perspective view of an exemplary cross point memoryarray 100 employing a single layer of memory. A bottom layer ofx-direction conductive array lines 105 is orthogonal to a top layer ofy-direction conductive array lines 110. The x-direction conductive arraylines 105 act as a first terminal and the y-direction conductive arraylines 110 act as a second terminal to a plurality of memory plugs 115,which are located at the intersections of the conductive array lines 105and 110. The conductive array lines 105 and 110 are used to both delivera voltage pulse to the memory plugs 115 and carry current through thememory plugs 115 in order to determine their resistive states.

Conductive array line layers 105 and 110 can generally be constructed ofany conductive material, such as aluminum, copper, tungsten or certainceramics. Depending upon the material, a conductive array line wouldtypically cross between 64 and 8192 perpendicular conductive arraylines. Fabrication techniques, feature size and resistivity of materialmay allow for shorter or longer lines. Although the x-direction andy-direction conductive array lines can be of equal lengths (forming asquare cross point array) they can also be of unequal lengths (forming arectangular cross point array).

FIG. 2A illustrates selection of a memory cell 205 in the cross pointarray 100. The point of intersection between a single x-directionconductive array line 210 and a single y-direction conductive array line215 uniquely identifies the single memory cell 205. FIG. 2B illustratesthe boundaries of the selected memory cell 205. The memory cell is arepeatable unit that can be theoretically extended in one, two or eventhree dimensions. One method of repeating the memory cells in thez-direction (orthogonal to the x-y plane) is to use both the bottom andtop surfaces of conductive array lines 105 and 110.

FIG. 1B depicts an exemplary stacked cross point array 150 employingfour memory layers 155, 160, 165, and 170. The memory layers aresandwiched between alternating layers of x-direction conductive arraylines 175, 180 and 185 and y-direction conductive array lines 190 and195 such that each memory layer 155, 160, 165, and 170 is associatedwith only one x-direction conductive array line layer and oney-direction conductive array line layer. Although the top conductivearray line layer 185 and bottom conductive array line layer 175 are onlyused to supply voltage to a single memory layer 155 and 170, the otherconductive array line layers 180, 190, and 195 can be used to supplyvoltage to both a top and a bottom memory layer 155, 160, 165, or 170.U.S. patent application, “Re-Writable Memory With Multiple MemoryLayers,” U.S. application Ser. No. 10/612,191, filed Jul. 1, 2003, nowissued U.S. Pat. No. 6,906,939, incorporated herein by reference in itsentirety for all purposes, describes stacked cross point arrays.

Referring back to FIG. 2B, the repeatable cell that makes up the crosspoint array 100 can be considered to be a memory plug 255, plus ½ of thespace around the memory plug, plus ½ of an x-direction conductive arrayline 210 and ½ of a y-direction conductive array line 215. Of course, ½of a conductive array line is merely a theoretical construct, since aconductive array line would generally be fabricated to the same width,regardless of whether one or both surfaces of the conductive array linewas used. Accordingly, the very top and very bottom layers of conductivearray lines (which use only one surface) would typically be fabricatedto the same size as all other layers of conductive array lines.

One benefit of the cross point array is that the active circuitry thatdrives the cross point array 100 or 150 can be placed beneath the crosspoint array, therefore reducing the footprint required on asemiconductor substrate. U.S. patent application, “Layout Of Driver SetsIn A Cross Point Memory Array,” U.S. application Ser. No. 10/612,733,filed Jul. 1, 2003, now issued U.S. Pat. No. 7,079,442, incorporatedherein by reference in its entirety for all purposes, describes variouscircuitry that can achieve a small footprint underneath both a singlelayer cross point array 100 and a stacked cross point array 150. Furtherdetails of the peripheral circuitry are described in U.S. patentapplication, “An Adaptive Programming Technique for a Re-WriteableConductive Memory Device,” U.S. application Ser. No. 10/680,508, filedOct. 6, 2003, now issued U.S. Pat. No. 6,940,744, incorporated herein byreference in its entirety for all purposes. FIG. 6A depicts x-directiondriver sets 605, 610, and 615 that are used to select specificx-direction conductive array lines in a X₀ layer 175, X₁ layer 180, andX₂ layer 185. Although the X₀ driver 605 and the X₂ driver 615 can useidentical logic, separate drivers are shown because of the difficulty inrouting the single X₀ driver 605 around a thru 650 that connects the X₁layer 180 to the X₁ driver 610. FIG. 6B depicts y-direction driver sets620 and 625 that are used to select specific y-direction conductivearray lines in the y-direction conductive array line layers 190 and 195.The Y₀ driver set 620 uses a thru 630 that goes through one ILD layer inorder to connect with the Y₀ layer 190. The Y₁ driver set 625 uses athru 635 that goes through three ILD layers in order to connect with theY₁ layer 195.

The cross point array is not the only type of memory array that can beused with a two-terminal memory element. For example, a two-dimensionaltransistor memory array can incorporate a two-terminal memory element.While the memory element in such an array would be a two-terminaldevice, the entire memory cell would be a three-terminal device.

FIG. 3 is a generalized diagrammatic representation of a memory cell 300that can be used in a transistor memory array. Each memory cell 300includes a transistor 305 and a memory plug 310. The transistor 305 isused to permit current from the data line 315 to access the memory plug310 when an appropriate voltage is applied to the select line 320, whichis also the transistor's gate. The reference line 325 might span twocells if the adjacent cells are laid out as the mirror images of eachother. U.S. patent application, “Non-Volatile Memory with a SingleTransistor and Resistive Memory Element,” U.S. application Ser. No.10/249,848, filed May 12, 2003, now issued U.S. Pat. No. 6,856,536,incorporated herein by reference in its entirety for all purposes,describes the specific details of designing and fabricating a transistormemory array.

The Memory Plug

Each memory plug 255 or 310 contains a multi-resistive state element(described later) along with any other materials that may be desirablefor fabrication or functionality. For example, the additional materialsmight include a non-ohmic device, as is described in application “HighDensity NVRAM,” U.S. application Ser. No. 10/360,005, filed Feb. 7,2003, now issued U.S. Pat. No. 6,917,539, incorporated herein byreference in its entirety for all purposes. The non-ohmic deviceexhibits a very high resistance regime for a certain range of voltages(IT_(NO−) to V_(NO+)) and a very low resistance regime for voltagesabove and below that range. The non-ohmic device, either alone or incombination with other elements, may cause the memory plug 255 or 310 toexhibit a non-linear resistive characteristic. Exemplary non-ohmicdevices include three-film metal-insulator-metal (MIM) structures andback-to-back diodes in series.

Furthermore, as described in “Rewriteable Memory With Non-Linear MemoryElement,” U.S. application Ser. No. 10/604,556, filed Jul. 30, 2003, nowissued U.S. Pat. No. 6,870,755, incorporated herein by reference in itsentirety for all purposes, it may also be possible for the memory cellexhibit non-linear characteristics without a separate non-ohmic device.It should be noted that since it is possible for a memory cell toexhibit non-linear characteristics the terms “resistive memory” and“resistive device” also apply to memories and devices showing non-linearcharacteristics, and can also be referred to as “conductive memory” and“conductive device.” While a non-ohmic device might be desirable incertain arrays, it may not be helpful in other arrays. Regardless, ifcertain treatments are used to improve the switching characteristics ofthe memory plug the treatments may also create an integrated non-ohmicdevice. Such a non-ohmic device may, therefore, be used even if it isnot necessary in that type of array.

Electrodes will typically be desirable components of the memory plugs255 or 310, a pair of electrodes sandwiching the multi-resistive stateelement. If the only purpose of the electrodes is as a barrier toprevent metal inter-diffusion, then a thin layer of metal, e.g. TiN, Pt,Au, Ag and Al could be used. However, conductive oxide electrodes mayprovide advantages beyond simply acting as a metal inter-diffusionbarrier. U.S. patent application, “Conductive Memory Device With BarrierElectrodes,” U.S. application Ser. No. 10/682,277, filed Oct. 8, 2003,now issued U.S. Pat. No. 7,067,862, incorporated herein by reference inits entirety for all purposes, describes electrodes (formed either witha single layer or multiple layers) that prevent the diffusion of metals,oxygen, hydrogen and water, act as a seed layer in order to form a goodlattice match with the conductive memory element, include adhesionlayers, and reduce stress caused by uneven coefficients of thermalexpansion, and provide other benefits. Additionally, the choice ofelectrode layers in combination with the multi-resistive state elementlayer may affect the properties of the memory plug 255 or 310, as isdescribed in U.S. patent application, “Resistive Memory Device With ATreated Interface,” U.S. application Ser. No. 10/665,882, filed Sep. 19,2003, now issued U.S. Pat. No. 7,326,979, incorporated herein byreference in its entirety for all purposes.

Typical electrodes 705, 715 and 725 (see FIG. 7) commonly used infabrication include Pt, Au, Ag and Al. If the only purpose of theelectrodes 705, 715 and 725 is as a barrier to prevent metalinter-diffusion, then a thin layer of metal, e.g. TiN, could be used.However, conductive oxide electrodes may provide advantages beyondsimply acting as a metal inter-diffusion barrier.

For example, a conducting oxide electrode might modify the formation andmigration of oxygen vacancies in the memory material. Oxygen vacanciescan cause degradation of electrical properties in the multi-resistivestate element 710 (see FIG. 7). A conducting oxide electrode can alsowithstand high temperature processing. Most metals either startoxidizing or combining with adjacent materials at temperatures above400° C. Accordingly, fabrication processes above these temperatures canbe considered to be high temperature processing. Additionally,conducting oxide electrodes will not degrade during operation. Regularmetal electrodes may degrade due to the electric field and interactionbetween the metal atoms and the memory material atoms.

Examples of conductive oxides include LaSrCoO₃, RuO₂, IrO₂, SrRuO₃,LaNiO₃ and doped strontium titanate (STO). The dopant used in STO can beeither Nb or Ta to substitute for titanium atoms, or any rare earth suchas La or Pr to substitute for strontium atoms. Generally, a conductingoxide electrode is metallic with resistivity below 1Ω-cm.

Conducting oxide electrodes can be fabricated directly, or can be madewith a material that is not initially an oxide, but is subsequentlyoxidized during further processing or operation. Ru and Ir are bothexamples of materials that can be oxidized during processing oroperation.

Additionally, certain materials oxidize at a finite rate and allow duallayers to form. For example, Ir might be particularly well suited formaking contact to an underlying conductive array line layer 105. When Iris oxidized, the top of the Ir layer becomes IrO₂. Since the IrO₂ growsat a finite rate it becomes possible to control the oxidation so that adual layer of Ir/IrO₂ is formed. Such a dual layer could provide a goodcontact on the un-oxidized bottom while still forming an oxygen barrieron the oxidized top.

Furthermore, some conductive oxides electrodes form a good lattice matchwith the multi-resistive state element 710, and thus lowercrystallization temperature for the resistive material. For example, ifthe multi-resistive state element 710 is STO, possible conductive oxideelectrodes that make a good lattice match include doped STO, LaSrCoO₃,and SrRuO₃. If the multi-resistive state element 710 is PCMO, possibleconductive oxide electrodes include the STO electrodes and also LaNiO₃.A seed layer will often be used on top of the thin layer of metal. Aseed layer will help the formation of the layer grown or deposited aboveit. For example, the seed layer could be on Pt, Ru, Ir or TiN. Some seedlayer/metal layer matches include LaNiO₃ or SrRuO₃ on Pt, IrO₂ on Ir,RuO₂ on Ru, and Pt on TiN.

Another benefit to certain conductive oxide electrodes is that stressmay be reduced by more closely matching the conductive oxide electrode'scoefficient of thermal expansion to the multi-resistive state element710.

The electrodes 705, 715 and 725 might be further improved by using alayer of metal such as platinum between the multi-resistive stateelement layer 710 and the conductive oxide electrode. Suchimplementations advantageously provide a good bather with the conductiveoxide, and a good contact with an adjacent metal layer.

Barrier layers are generally helpful to prevent inter-diffusion of atomsafter different materials have been deposited. For example, barrierlayers can block the diffusion of metals, oxygen, hydrogen or water.Binary oxides or nitrides with 2 elements and ternary oxides or nitrideswith 3 elements are particularly suited to high temperature processing.Unlike a regular electrode like titanium that oxidizes and becomesnon-conductive, titanium nitride will not oxidize and will remainconductive until about 500° C. Ternary oxides oxidize at even highertemperatures, typically about 50° C. higher than binary oxides. The rateof oxidation depends on the temperature and the oxygen partial pressure.

Examples of binary nitrides include titanium nitride, tantalum nitrideand tungsten nitride. Examples of ternary nitrides include titaniumsilicon nitride, tantalum aluminum nitride, tantalum silicon nitride,and ruthenium titanium nitride. An example of a ternary oxide isruthenium tantalum oxide.

As will be appreciated by those skilled in the art, an electrode mayrequire other layers, in order to properly function. For exampleadhesion layers are sometimes necessary. An adhesion layer is usedbetween a substrate and thin-film layer to improve adhesion of thethin-film layer to substrate. Pt does not stick well to SiO₂, so a gluelayer, such as Ti or TiO₂, is used between them for better adhesion.Similarly, a sacrificial barrier layer is an oxide layer that isdeposited for the sole purpose of capturing all the oxygen that couldotherwise diffuse into other layers, such as the multi-resistive stateelement 710. The electrode 705 is considered to consist of everything inbetween x-direction conductive array line 210 and the multi-resistivestate element 710, including any adhesion or sacrificial barrier layers,as required. Similarly, the electrode 715 consists of all layers betweenthe multi-resistive state element 710 and the non-ohmic device 720 andthe electrode 725 consists of everything in between the non-ohmic device720 and the y-direction conductive array line 215.

For example, an electrode may include a TiN or TiAlN layer, an Ir layerand an IrO₂ layer to have good metal bather and oxygen barrierproperties. However, such additional layers are only necessary to theextent they are required. Certain conductive oxide electrodes mayprovide multiple functions. For example, ternary nitrides and ternaryoxides that have one component that is either ruthenium or iridium andanother component that is either tantalum or titanium can act as both abarrier layer and a sacrificial high-temperature oxygen barrier.

It will be appreciated that the choice of electrode layers 705, 715 and725 in combination with the multi-resistive state element layer 710 mayaffect the properties of the memory plug 255 or 310.

The multi-resistive state element will generally (but not necessarily)be crystalline—either as a single crystalline structure or apolycrystalline structure. One class of multi-resistive state elementare perovskites that include two or more metals, the metals beingselected from the group consisting of transition metals, alkaline earthmetals and rare earth metals. The perovskites can be any number ofcompositions, including manganites (e.g., Pr_(0.7)Ca_(0.3)MnO₃,Pr_(0.5)Ca_(0.5)MnO₃ and other PCMOs, LCMOs, etc.), titanates (e.g.,STO:Cr), zirconates (e.g., SZO:Cr), other materials such as Ca₂Nb₂O₇:Cr,and Ta₂O₅:Cr, and high Tc superconductors (e.g., YBCO). Specifically,MnO₃, when combined with the rare earth metals La, Pr or somecombination thereof and the alkaline earth metals Ca, Sr or somecombination thereof have been found to produce a particularly effectivemulti-resistive state element for use in the memory plug 255 or 310. Thecompounds that make up the perovskite class of multi-resistive stateelements include both simple conductive metal oxides and complexconductive metal oxides. Further, some oxides that may not be conductivein their pure form may be used as they become conductive through theaddition of dopants, or if they are used as a very thin layer (e.g., inthe order of tens of Angstroms) in which case tunneling conduction canbe achieved. Therefore, as will be appreciated by those skilled in theart, the terms “conductive memory,” “conductive element,” and“conductive device” can include devices that are fabricated withmaterials that are classified as insulators, but are thin enough toallow tunneling conduction.

Multi-resistive state elements, however, are not limited to perovskites.Specifically, any conductive element (composed of either a singlematerial or a combination of materials) that has a hysteresis thatexhibits a resistive state change upon application of a voltage whileallowing non-destructive reads is a good candidate for a multi-resistivestate element. A non-destructive read means that the read operation hasno effect on the resistive state of the memory element. Measuring theresistance of a memory cell is accomplished by detecting either currentafter the memory cell is held to a known voltage, or voltage after aknown current flows through the memory cell. Therefore, amulti-resistive state element that is placed in a high resistive stateR₀ upon application of −V_(W) and a low resistive state R₁ uponapplication of +V_(W) should be unaffected by a read operation performedat −V_(R) or +V_(R). In such materials a write operation is notnecessary after a read operation. The same principle applies if morethan one resistive state is used to store information (e.g., themulti-resistive state element has a high resistive state of R₀₀, amedium-high resistive state of R₀₁, a medium-low resistive state of R₁₀and a low resistive state of R₁₁).

As described in U.S. patent application, “A 2-Terminal Trapped ChargeMemory device with Voltage Switchable Multi-Level Resistance,” U.S.application Ser. No. 10/634,636, filed Aug. 4, 2003, now issued U.S.Pat. No. 7,038,935, incorporated herein by reference in its entirety forall purposes, trapped charges are one mechanism by which the hysteresiseffect is created. Trapped charges can be encouraged with dopants, asdescribed in U.S. patent application, “Multi-Resistive State Materialthat Uses Dopants,” U.S. application Ser. No. 10/604,606, filed Aug. 4,2003, now issued U.S. Pat. No. 7,071,008, incorporated herein byreference in its entirety for all purposes.

Treating the Conductive Element

Properties of the multi-resistive state elements can be furtherenhanced, or even created, with certain treatments. For example, areactive metal, such as Al, Ti, Mg, W, Fe, Cr, V, Zn, Ta or Mo cancreate a differential between the high resistive state and the lowresistive state in a conductive metal oxide that does not exhibitswitching properties in an untreated condition. Similarly, a reactivemetal can enhance the switching properties of a conductive element thatalready exhibits switching properties. The reactive metal reacts withthe conductive element and forms a layer of reacted metal, thereforecreating a multi-resistive state element with enhanced properties.Furthermore, following the deposition of the reactive metal with ananneal (e.g., 400° C. in an argon (Ar) or similarly non-reactiveenvironment) can give the memory plug a more stable structure.

The reactive metal can additionally create a non-ohmic device within themulti-resistive state element. Typically, the thicker the layer ofreacted metal is, the greater the range of voltages V_(NO−) to V_(NO+).However, there is a limit to how much reactive metal can diffuse andreact with the conductive element.

For example, if 500 Å of Al were used on a PCMO perovskite, only thefirst 100 Å may fully react with the underlying conductive element. Theunreacted portion of the reactive metal might then cause the memory plugto have degraded properties. Furthermore, the best switching propertiesmay not coincide with the largest range of voltages V_(NO−) to V_(NO+).Although most applications would use between 10 Å and 100 Å of reactivemetal, between 25 Å and 50 Å would typically be preferred in mostconditions. Those skilled in the art will appreciate that such reactivemetal layers would be considered “very thin” layers. Very thin layerstypically describe any layer that is less than 200 Å.

By monitoring current through a resistance set in series with themulti-resistive state element, and graphing this current versus anapplied pulse shaped as a double ramp, an I-V curve can be obtainedwhich shows the switching of the memory element in real time. Thiscontrasts with standard I-V curves, which are taken with a very slowramp, in the order of tens of millisecond, and only represent the DCfunctionality of the memory element. FIG. 5A is a chart of measurementsacross a multi-resistive state memory element after a layer of reactivemetal has been deposited, using a pulse with a 10 μs positive ramp, 5 μshigh or low level at +3.5 and −4V, and a 10 μs negative ramp, as shownin FIG. 5B.

FIG. 5C graphically depicts one example of a non-linear I-Vcharacteristic 500 for a discrete re-writeable non-volatile two-terminalresistive memory element (e.g., memory element 255, 310, 710) havingintegral selectivity due to its non-linear I-V characteristics and thenon-linear I-V characteristic is maintained regardless of the value ofthe data stored in the memory cell, that is the I-V characteristic ofthe memory element does not change from non-linear to linear as afunction of the resistive state stored in the memory element. Therefore,the non-linear I-V characteristic of the memory element is non-linearfor all values of stored data (e.g., resistive states). Voltage Vapplied across the memory element is plotted on the Y-axis and currentdensity J through the memory element is plotted on the X-axis. Here,current through the memory element is a non-linear function of theapplied voltage across the memory element. Accordingly, when voltagesfor data operations (e.g., read and write voltages) are applied acrossthe memory element, current flow through the memory element does notsignificantly increase until after a voltage magnitude of about 2.0V(e.g., at 0.2 A/cm²) is reached (e.g., a read voltage of about 2.0Vacross the memory element). An approximate doubling of the voltagemagnitude to about 4.0V does not double the current flow and results ina current flow of ≈0.3 A/cm². The graph depicted is only an example andactual non-linear I-V characteristics will be application dependent andwill depend on factors including but not limited to an area of thememory element (e.g., area determines the current density J) and thethin-film materials used in the memory element, just to name a few. Thearea of the memory element will be application dependent. Here, thenon-linear I-V characteristic of the discrete memory element applies toboth positive and negative values of applied voltage as depicted by thenon-linear I-V curves in the two quadrants of the non-linear I-Vcharacteristic 500. One advantage of a discrete re-writeablenon-volatile two-terminal resistive memory element that has integralselectivity due to a non-linear I-V characteristic is that when thememory element is half-selected (e.g., one-half of the magnitude of aread voltage or a write voltage is applied across the memory element)during a data operation to a selected memory cell(s), the non-linear I-Vcharacteristic is operative as an integral quasi-selection device andcurrent flow through the memory element is reduced compared to a memorycell with a linear I-V characteristic. Therefore, a non-linear I-Vcharacteristic can reduce data disturbs to the value of the resistivestate stored in the memory element when the memory element isun-selected or is half-selected. In other embodiments, the memoryelement (e.g., memory element 100, 200) has a non-linear I-Vcharacteristic for some values of the resistive state stored in thememory element and a linear I-V characteristic for other values of theresistive state stored in the memory element.

Fabrication

FIG. 4 is an exemplary flow chart of various processing steps that couldbe involved in fabrication of a memory. At 405, standard front end ofline (FEOL) processes can be used to form the active circuitry thatdrives the cross point memory array. FEOL processes are generallydefined as operations performed on a semiconductor wafer in the courseof device manufacturing up to first metallization, and might end withchemical-mechanical polishing (CMP) of an inter-layer dielectric (ILD).Certain cross point arrays, especially those with active circuitryunderneath the memory array, might also include various metallizationlayers in step 405. The metallization layers are used to electricallyconnect the active circuitry to the conductive array lines of the crosspoint array 100 or 150.

The next processing step at 410 is formation of contact holes throughthe ILD to appropriate positions in the active circuitry (ormetallization layers in the case of some cross point arrays) followed byplug formation at 415. Certain transistor memory arrays may requirethese steps if, for example, the memory plug 310 were so wide that itwould overlap the transistor's gate 320. Otherwise, the memory plug 310could be formed directly on top of the semiconductor substrate 305.

Once the plugs are formed, a cross point array 100 or 150 would requirethat the conductive array lines be patterned on the wafer at 420. Ifrefractory metals with relatively high resistivities are used for theconductive array lines, the maximum length and minimum cross-sectionalarea may be limited in comparison to aluminum or copper.

Another ILD layer could be deposited over the first layer of conductivearray lines at 425. The dielectric layer can be deposited over theconductive array lines by plasma-enhanced chemical vapor deposition(PECVD) and then planarized by CMP to expose the top surfaces of theconductive array lines.

At 430 the memory plug formation begins. In the case of transistormemory array, the memory plug can be formed directly on the contact holeplugs. In the case of a cross point array, the memory plugs are formedon the bottom conductive array lines.

Regardless of the memory array, a memory plug generally begins with thedeposition of the bottom electrodes at 430. At 435 the multi-resistivestate elements are deposited, typically using high temperatureprocessing (e.g., solution based spin on followed by high temperatureanneal, pulsed laser deposition, sputtering, and metal-organic chemicalvapor deposition). However, U.S. patent applications, “Laser Annealingof Complex Metal Oxides (CMO) Memory Materials for Non-Volatile MemoryIntegrated Circuits,” U.S. application Ser. No. 10/387,799, now issuedU.S. Pat. No. 7,309,616, and “Low Temperature Deposition of ComplexMetal Oxides (CMO) Memory Materials for Non-Volatile Memory IntegratedCircuits,” U.S. application Ser. No. 10/387,773, now issued U.S. Pat.No. 7,063,984, both filed Mar. 13, 2003, and both incorporated herein byreference in their entireties for all purposes, describe fabricationtechniques that may be able to be used in lieu of high temperaturefabrication processes. If high temperature fabrication were used, thenall the circuitry elements that were deposited before themulti-resistive state element would need to withstand those hightemperatures. Using refractory metals are one technique that can be usedto create elements that can endure high temperatures.

It should also be appreciated that fabrication of the multi-resistivestate element might include additional techniques in order to ensure aneffective memory device. For example, biasing the multi-resistive stateelement might be beneficial in order to ensure the hysteresis ispresented in a certain direction. U.S. patent application, “Multi-LayerConductive Memory Device,” U.S. application Ser. No. 10/605,757, filedOct. 23, 2003, now issued U.S. Pat. No. 6,965,137, incorporated hereinby reference in its entirety for all purposes describes using amulti-layered multi-resistive state element in order to encourage ahysteresis in a certain direction. As previously discussed, a reactivemetal can also be a desirable addition to the multi-resistive stateelement.

At 440 another electrode is deposited on top of the multi-resistivestate element. At 450 the optional non-ohmic device is formed. If thenon-ohmic device is a MIM structure, a top electrode layer may or maynot be necessary at 455. In addition, this top electrode layer couldinclude a barrier layer to prevent metal inter-diffusion.

At 460 standard photolithography and appropriate multi-step etchprocesses could be used to pattern the memory/non-ohmic film stack intomemory cell plugs. U.S. patent application, “Conductive Memory StackWith Non-Uniform Width,” U.S. application Ser. No. 10/605,963, nowissued U.S. Pat. No. 7,009,235, filed Nov. 10, 2003, incorporated hereinby reference in its entirety for all purposes describes an improvedfabrication technique that includes etching a memory plug with anon-uniform width and using a sidewall layer around the memory plug.

At 465 depositing another ILD, which can then be planarized by CMP,fills in the spaces between the plugs. At 470 via holes are formed inthe ILD. Via holes could be formed to connect the tops of the memorycell islands and are one mechanism that can be used to provideconnections between metal interconnect layers. The via holes are thenfilled at 475.

The top layer(s) of conductive array lines could then be formed at 480.If there are no more memory elements to form at high temperature, thefinal layer(s) of conductive array lines may comprise aluminum, copperor other high conductivity metal.

Concluding Remarks

Although the invention has been described in its presently contemplatedbest mode, it is clear that it is susceptible to numerous modifications,modes of operation and embodiments, all within the ability and skill ofthose familiar with the art and without exercise of further inventiveactivity. For example, instead of limiting how much reactive metal isdeposited on top of the multi-resistive state element, any excessunreacted material can simply be polished off with CMP. Accordingly,that which is intended to be protected by Letters Patent is set forth inthe claims and includes all variations and modifications that fallwithin the spirit and scope of the claim.

What is claimed is:
 1. A re-writeable non-volatile memory device,comprising: a re-writeable non-volatile two-terminal memory element (ME)comprising tantalum, the ME including a first terminal, a secondterminal, a first layer of a conductive metal oxide (CMO), and a secondlayer in direct contact with the first layer, the second layer and thefirst layer operative to store at least one-bit of data as a pluralityof resistive states, the first and second layer are electrically inseries with each other and with the first and second terminals, whereinthe first layer includes a first lattice structure and the second layerincludes a second lattice structure that substantially matches the firstlattice structure.
 2. The re-writeable non-volatile memory device ofclaim 1, wherein the second layer comprises Al, Ti, Mg, W, Fe, Cr, V,Zn, Ta or Mo.
 3. The re-writeable non-volatile memory device of claim 1,wherein the second layer comprises reactive metal or has a thicknessthat is approximately 200 Angstroms or less.
 4. The re-writeablenon-volatile memory device of claim 1 and further comprising: anintegral non-ohmic device (NOD) created by the direct contact betweenthe first and second layers, the integral NOD is electrically in serieswith the ME and with the first and second terminals.
 5. A re-writeablenon-volatile memory device, comprising: a re-writeable non-volatiletwo-terminal memory element (ME) comprising tantalum, the ME including afirst terminal, a second terminal, a first layer of a conductive metaloxide (CMO), and a second layer in direct contact with the first layer,the second layer and the first layer operative to store at least one-bitof data as a plurality of resistive states, the first and second layerare electrically in series with each other and with the first and secondterminals, wherein the direct contact between the first layer and thesecond layer is operative to impart a non-linear I-V characteristic tothe ME.
 6. The re-writeable non-volatile memory device of claim 5,wherein the ME includes the non-linear I-V characteristic for both aprogramming voltage applied across the first and second terminals and anerase voltage applied across the first and second terminals.
 7. There-writeable non-volatile memory device of claim 5, wherein the MEincludes the non-linear I-V characteristic that is non-linear in both apositive quadrant and a negative quadrant of the non-linear I-Vcharacteristic.
 8. The re-writeable non-volatile memory device of claim5 and further comprising: a non-ohmic device (NOD) electrically inseries with the ME and the first and second terminals.
 9. There-writeable non-volatile memory device of claim 5, wherein the tantalumis of a compound, wherein the compound comprises tantalum oxide.
 10. There-writeable non-volatile memory device of claim 9, wherein the tantalumoxide comprises at least one of tantalum pentoxide or ruthenium tantalumoxide.
 11. The re-writeable non-volatile memory device of claim 5,wherein the tantalum is of a ternary oxide, the ternary oxide comprisingat least one of ruthenium or iridium.
 12. The re-writeable non-volatilememory device of claim 5, wherein at least a portion of the tantalum ofthe ME is located in the first layer.
 13. The re-writeable non-volatilememory device of claim 5, wherein at least a portion of the tantalum ofthe ME is located in the second layer.
 14. A re-writeable non-volatilememory device, comprising: a re-writeable non-volatile two-terminalmemory element (ME) comprising tantalum, the ME including a firstterminal, a second terminal, a first layer of a conductive metal oxide(CMO), and a second layer in direct contact with the first layer, thesecond layer and the first layer operative to store at least one-bit ofdata as a plurality of resistive states, the first and second layer areelectrically in series with each other and with the first and secondterminals, wherein the re-writeable non-volatile two-terminal memoryelement is positioned between a cross-point of a first conductive arrayline with a second conductive array line, the re-writeable non-volatiletwo-terminal memory element includes a first terminal directlyelectrically coupled with the first conductive array line and a secondterminal directly electrically coupled with the second conductive arrayline, and wherein the re-writeable non-volatile two-terminal memoryelement is directly electrically in series with its respective first andsecond conductive array lines.
 15. The re-writeable non-volatile memorydevice of claim 14, wherein the CMO comprises a perovskite.
 16. There-writeable non-volatile memory device of claim 14 and furthercomprising: a two-terminal cross-point array including: a plurality ofthe first conductive array lines, a plurality of the second conductivearray lines, and a plurality of the re-writeable non-volatiletwo-terminal memory elements, and wherein each re-writeable non-volatiletwo-terminal memory element is positioned between the cross-point of oneof the plurality of the first conductive array lines with one of theplurality of the second conductive array lines.
 17. The re-writeablenon-volatile memory device of claim 16, wherein the plurality of firstand second conductive array lines are electrically coupled withfront-end-of-the-line (FEOL) active circuitry fabricated on asemiconductor substrate, the two-terminal cross-point array is incontact with the semiconductor substrate and is fabricated directlyabove the semiconductor substrate, and at least a portion of the FEOLactive circuitry configured to access at least one of the re-writeablenon-volatile two-terminal memory elements for read operations, writeoperations, or both read and write operations.
 18. The re-writeablenon-volatile memory device of claim 17, wherein the two-terminalcross-point array comprises a stacked cross-point array including aplurality of memory layers.
 19. The re-writeable non-volatile memorydevice of claim 3, wherein the second layer has a thickness that is inthe order of tens of Angstroms.